Problem: Solve for $a$ : $a + 21 = 19$
Answer: Subtract $21$ from both sides: $ \begin{eqnarray} \\ a + 21 &=& 19 \\ \\ {-21} && {-21} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{21 + a} &\hphantom{=}& \hphantom{19} \\ a &=& 19 {- 21} \\ \end{eqnarray} $ Simplify. $ a = -2$